1. Field
Apparatuses consistent with exemplary embodiments relate to a motor used to convert electric energy into mechanical energy for various fields including electric automobile and home appliance.
2. Description of the Related Art
Hybrid vehicles and electric vehicles have gained more popularity due to harmful environmental effects from the air pollution and an increasing shortage of fossil fuels. A hybrid vehicle mainly uses an internal-combustion engine to generate power and uses an electric motor as an auxiliary power source. An electric vehicle uses an electric motor as a main power source.
With the development of technologies for a battery and a motor, it is expected that electric vehicles, known as pollution-free cars, will substitute for “transition” vehicles, such as hybrid cars, since electric vehicles emit no pollutants or carbon dioxide while driving.
There are various requirements for operating a motor for the electric vehicle. When an electric vehicle ascends a hill, low-speed high-torque is required, whereas during high-speed driving, for example, on an expressway, high-speed low torque is required. In the case of a washing machine, for a washing operation with a tub loaded with laundry and water, low-speed high-torque is required, whereas high-speed low-torque is required for draining water from the tub.
The use of a permanent magnet synchronous motor has been recently increasing for use with electric automobiles and washing machines. A permanent magnet synchronous motor uses a permanent magnet for a rotor. When a maximum current equivalent to a current limit of the permanent magnet synchronous motor flows through a stator coil in a q-axis direction that is 90 degrees ahead of a d-axis of the permanent magnet of the rotor, it is possible to provide low-speed high-torque at high efficiency.
However, because during a high-speed operation, a counter electromotive force due to the permanent magnet exceeds a voltage limit of the motor, field weakening control is conducted in an electrical manner, whereby a q-axis current is reduced and a −d-axis current is fed to reduce air gap flux density of the motor. During such control process, the −d-axis current does not merely contribute to generation of motor torque, but only increases copper loss.